Silicon-germanium (SiGe) has become another important semiconductor material other than silicon (Si) and gallium arsenide (GaAs). SiGe has more excellent properties than pure Si material, and the production processes of SiGe are compatible with Si processes. SiGe HBT has electrical properties which are almost in the same level as those of the same kind of devices made of compound semiconductor material like GaAs. The SiGe HBT has a broad application prospect in RF (radio-frequency), especially in ultra-high frequency field. Moreover, the SiGe HBT can be integrated with CMOS process, which gives full play to the advantages of CMOS process in high integration and low cost and meanwhile realizes high-frequency and low-noise properties of SiGe/Si HBT.
However, a SiGe HBT or a BiCMOS could have some parasitic effects such as collector-substrate and collector-base capacitances, which could harm the high-frequency characteristics of the device. Therefore, reduction of parasitic capacitances will contribute to the improvement of properties of HBT devices.